vdpa/octeon_ep: Fix error code in octep_process_mbox()

Return -EINVAL for invalid signatures.  Don't return success.

Fixes: 8b6c724c ("virtio: vdpa: vDPA driver for Marvell OCTEON DPU devices")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Message-Id: <623e885b-1a05-479e-ab97-01bcf10bf5b8@stanley.mountain>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/octeon_ep/octep_vdpa_hw.c

@@ -140,7 +140,7 @@ static int octep_process_mbox(struct octep_hw *oct_hw, u16 id, u16 qid, void *bu
 	val = octep_read_sig(mbox);
 	if ((val & 0xFFFF) != MBOX_RSP_SIG) {
 		dev_warn(&pdev->dev, "Invalid Signature from mbox : %d response\n", id);
-		return ret;
+		return -EINVAL;
 	}
 
 	val = octep_read_sts(mbox);